1. Field of the Invention
The present invention relates to the field of animal healthcare, particularly to a novel porcine circovirus type 2 (PCV2) strain, an immunogenic composition containing the virus strain, PCV2 test kit, and application of the virus strain.
2. Description of the Prior Art
Porcine circovirus (PCV) was first discovered in a pig kidney cell line (PK-15, ATCC CCL33). Although the porcine circovirus can continuously contaminate PK-15 cells, the virus does not cause cytopathic effect (CPE) in the contaminated PK-15 cells. In addition, the PK-15-derived PCV is considered apathogenic. Even though the porcine circovirus can infect pigs, it does not cause lesions in the infected pigs. The porcine circovirus is an icosahedron-shaped, single-stranded DNA virus with a circular genome of 1,759 base pairs (bp). The PK-15-derived PCV was classified in the Circoviridae family in 1995.
In 1991, post-weaning multisystemic wasting syndrome (PMWS) was first recorded in pigs in Canada. PMWS affects 5- to 12-week-old piglets. It is clinically characterized by such symptoms as progressive weight loss, dyspnoea, pallor, and occasional jaundice. Its characteristic microscopic lesions include lymphoid depletion with histiocytic infiltrates within lymphoid organs, pneumonia, granulomatous inflammation, hepatitis, and nephritis. Since the 1991 outbreak in Canada, PMWS has been reported in many countries in North America, Europe, and Asia. Later, a new circovirus was isolated from pigs with PMWS. Whereas the morphology of the PMWS-derived porcine circovirus is very similar to that of the PK-15-derived PCV, the two porcine circoviruses share only 68 to 76% homology in their genome sequences. The apathogenic PK-15-derived PCV and the pathogenic PMWS-derived PCV were further identified as porcine circovirus type 1 (PCV1) and porcine circovirus type 2 (PCV2), respectively.
Porcine circovirus type 2 (PCV2) also belongs to the Circoviridae family. PCV2 is a nonenveloped, icosahedrons-shaped, single-stranded, circular DNA virus with a diameter of 17 nm, known to be one of the smallest animal viruses. The PCV2 circular genome contains the origin of replication with a stem loop structure, which is a common characteristic of circoviruses. The PCV2 genome comprises 1,767 or 1,768 nucleotides and is assumed to have 11 potential open reading frames (ORFs). Among the 11 ORFs, ORF 1 and ORF 2 are probably the most important genes, which encode replication (Rep and Rep′) proteins and a capsid (Cap) protein, respectively. The capsid (Cap) protein encoded by PCV2 ORF2 gene would most likely be the antigen that induces production of neutralizing antibodies.
PCV2 infection is widespread on most of the pig farms in the world. PCV2 has been found to cause low survival rates and low feed conversion rates (FCRs) of infected pigs and result in great economic losses in pig farming. Therefore, it is important to develop PCV2 test kits and vaccine against PCV2.